1. Field of the Invention
The present invention relates to wireless communications, and more particularly to a technique for selecting an access point in a wireless network.
2. Description of Related Art
Consumer demand for high-speed access to Internet and intranet related services and applications has resulted in several high-bandwidth access network alternatives, such as DSL (Digital Subscriber Line) broadband networks, all fiber networks, ISDN (Integrated Services Digital Network), and fixed wireless networks.
Fixed wireless provides a viable alternative to traditional wire-based access, particularly in geographic regions where the costs of upgrading and maintaining wireline connections are high. Essentially, a fixed wireless network is a cellular network which relies on short-range transmitter/receiver (xe2x80x9ctransceiverxe2x80x9d) base stations to serve subscribers in small regions (xe2x80x9ccellsxe2x80x9d) of a larger service area. By dividing a service area into cells with limited range transceivers, the same frequencies can be reused in different regions of the service area, and subscriber terminals which consume relatively little power can be used to communicate with a serving base station.
FIG. 1 illustrates a conventional wireless internet access system (WIAS), which is one specific implementation of fixed wireless technology having four major components: (1) multiple data base stations (BS) 100(a) and 100(b) which provide wireless connectivity and radio coverage to subscriber units 102(a)-(d) (for example, residential and corporate terminal equipment as illustrated in FIG. 1); (2) wireless modems (xe2x80x9cWMsxe2x80x9d) 170(a)-(c) which allow the subscriber units 102(a)-(d) to communicate with BS 100(a) or 100(b) via forward (base station to subscriber) and reverse (subscriber to base station) air-interface links 115(a)-(c); (3) a data switching center (DSC) 125 for routing data packets to/from BS 100(a) and 100(b); and (4) a backbone transmission network 135, such as public IP (Internet Protocol) network, connected to the DSC 125.
Subscriber units may connect to the backbone transmission network 135 in various ways, examples of which are shown in FIG. 1. Corporate terminals 102(c) and 102(d) are connected to the backbone transmission network 135 via a local area network (LAN), a wireless router and/or firewall (not shown), and a shared WM 170(c), while subscriber units 102(a) and 102(b) each have their own dedicated WM 170(a), 176(b). BS 100(a) and 100(b) may be directly connected to the DSC 125 or communicate with the DSC 125 via a service provider""s private IP network 127.
FIG. 2 illustrates an exemplary cell pattern suitable for implementing fixed wireless access. As seen in FIG. 2, each BS 100(a) and 100(b) provides 360xc2x0 RF service coverage to subscriber terminals in cells 150(a) and 150(b), respectively, by transmitting and receiving signals over air-interface channels in designated frequency blocks (e.g., 5 MHz wide transmit frequency blocks and 5 MHz receive frequency blocks). Typically, cell coverage is sectorized, such that the frequency block designated for a given cell is distributed among a plurality of sectors (e.g., for a five sector per cell configuration, each sector being assigned a 1 MHz block for transmitting and a 1 MHz block for receiving). Therefore, each BS 100(a) and (b) includes a plurality of access points (xe2x80x9cAPsxe2x80x9d, not shown in FIG. 1), one per sector.
Depending on the location of a subscriber""s WM relative to cell/sector boundaries and the radio frequency (RF) propagation characteristics of the surrounding area, the subscriber may be capable of communicating with multiple APs, i.e., multiple APs for a single cell and/or APs from different cells. For example, a subscriber""s WM may be at or near the boundary of two or more sectors and/or two or more cells. In present implementations of fixed wireless access, the installer of the subscriber""s WM selects a single AP during setup based on forward link signal strength, and the assignment of the AP which transmits/receives to/from the subscriber""s WM does not change.
Due to changing RF propagation characteristics of the surrounding area, however, the AP which provides the best performance during installation will often not always be the best or even a suitable AP for ensuring adequate service quality or data throughput rates. For example, temperature and climatic changes, particularly moisture levels which change reflection coefficients, can significantly affect RF propagation between the AP and a subscriber""s WM. Furthermore, degradation of service may result if the AP assigned to the user temporarily fails, or the sector served by the AP becomes overloaded. Still further, a more suitable AP may be subsequently deployed by the service provider (e.g., as a result of growth and xe2x80x9ccell-splittingxe2x80x9d).
Therefore, the need exists for an agile AP selection and assignment technique which allows a subscriber""s WM to select and switch between serving APs in response to network conditions.
The present invention is a method and apparatus for selecting an access point in a wireless communications network which is able to maintain adequate service quality and throughput rates under dynamic network conditions. In one embodiment, the present invention is a technique for selecting and assigning an access point in a fixed wireless network which monitors control signals transmitted by a plurality of neighboring access points and selects the best access point as a function of a communication link quality metric, such as signal strength at the user location, signal quality at the user location, signal strength at the access point, signal quality at the access point, packet error rate (xe2x80x9cPERxe2x80x9d) at the wireless modem, PER at the access point, or a combination of two or more of these measurements, and relative sector load levels.
By dynamically selecting the best serving access point as a function of the communication link quality metric and relative load levels, the subscriber""s wireless modem is able to adapt to changing RF propagation characteristics of the surrounding area to maintain high service quality and throughput rates, and is also able to avoid service outages by directing traffic away from a failed or overloaded access point.
In one exemplary implementation, a wireless modem in a fixed wireless network executes an AP search/selection sequence in response to a triggering event, such as when the wireless modem is initially powered-up, when service quality degrades below a threshold level, when sector load exceeds a threshold, or when instructed by the serving AP to do so, to (re)select an access point. When a triggering event has occurred, the subscriber""s wireless modem detects an access control signal, commonly referred to as a xe2x80x9cbeaconxe2x80x9d, transmitted from a plurality of neighboring access points. An access point""s beacon identifies the access point, includes a neighbor list to identify neighboring access points and the frequency channel on which such neighboring access points are transmitting, and includes a field which indicates the access point""s load level. After detecting beacons and obtaining a communication link quality metric for each neighboring access point, the wireless modem selects the best access point based on the communication link quality metric and relative load levels.
By considering relative load levels, adequate throughput rates can be maintained by distributing service among a greater number of access points when possible (i.e., achieving load balancing). Furthermore, by initiating access point re-selection when service quality degrades below a threshold level, or when sector load exceeds a threshold, the subscriber""s wireless modem is able to react to changes in RF propagation conditions in the surrounding area, such as temperature and other climatic changes which affect communication quality.